Minimum delay communication in energy harvesting systems over fading channels

We consider an energy harvesting wireless system where data is stored in a data buffer and energy is harvested from various sources and stored in an energy buffer. The transmitter tries to minimize a delay criterion, e.g., average delay in the data buffer or probability of overflow, subject to the hard power constraints imposed by the available energy. We show that if the average rate of data arrival is larger than a certain threshold, then the probability of overflow is lower bounded by a constant and the average delay is infinite, and if it is smaller than this threshold, then there exists a sequence of simple policies such that the probability of overflow goes to zero faster than 1/L^K for any K ≥ 2, where L is the length of the data buffer, and average delay is upper bounded by some constants. Furthermore, we extend the results into a multiple access channel (MAC) with two users, and study two situations, when the energy buffers of the two users are separate, and when the energy buffers of the two users can cooperate.